Non-invasive techniques for measurement of various blood parameters, such as blood oxygen saturation and the concentration of substances contained in the blood, have become very popular, since they advantageously do not require the withdrawal of a blood sample from a patient's body. Optical monitoring techniques of the kind specified typically utilize the detection of light transmitted or reflected from the location on the patient's body under measurement. Most of the known techniques utilize a measurement optical device or probe, designed in a manner to be attached to the patient's finger, which includes an optical assembly for irradiating the finger with light and detecting its light response.
U.S. Pat. No. 5,810,723 discloses an apparatus for the non-invasive monitoring of a patient's carboxyhemoglobin level. The patient breathes oxygen to saturate his blood hemoglobin prior to detection. The apparatus utilizes a clamp with arms which hold the patient's finger: one arm supports a light emitting source and the other supports a detector. A microprocessor controls the measurements and processes the detected signals.
U.S. Pat. No. 5,638,816 and its continuation, U.S. Pat. No. 5,860,919, disclose an apparatus for the non-invasive monitoring of blood parameters by applying pressure to the patient's finger, thus inducing an active pulse therein. The induced change of blood volume enables a better signal-to-noise ratio to be obtained.
U.S. Pat. No. 5,782,757 discloses a measuring devices in the form of disposable, folded adhesive sensors with optics embedded therein. The probe is designed so as to fit comfortably onto a patient's fingertip.
All the conventional devices of the kind specified are aimed at measuring enhanced optical pulsatile signals caused by the changes in the volume of the blood containing medium (finger). It is known that a regular optical pulsatile signal is typically 2-3% of the total transmission. The above devices are capable of obtaining the enhanced pulsatile signal that reach 8-10% of the total light transmission intensity. This enhancement of the natural pulsatile signal is a boundary of all conventional techniques of the kind specified.
A different technique is disclosed in co-pending application PCT/IL 99/00331, assigned to the assignee of the present application, where the measured signals are not pulsatile. This is an occlusion based technique, according to which a state of blood cessation is created in a medium under measurement and measurements are taken during this state. This enables to obtain significantly enhanced light response of the medium, as compared to that of the previously described techniques dealing with the pulsatile signals. To create such a state of blood cessation, over-systolic pressure should be applied to the patient's finger at either location upstream of the area under measurement, with respect to the direction of normal blood flow. None of the conventional probes is suitable for this purposes.